Steering power assistance pumps are usually in the form of vane-type pumps and are rigidly connected to the drive engine of the motor vehicle in which the steering assistance system is used. Accordingly the pump delivery flow increases with increasing engine speed. However, there is generally no need for a strong steering power assistance effect when the engine is rotating at high speeds. For that reason, the systems generally use a flow control valve for bypassing a part of the pump delivery flow while the remaining regulated output flow is taken back to the tank by way of the steering valve. When that happens, the hydraulic fluid which is under what is referred to as dynamic pressure experiences release of pressure which results in a waste of energy, unless the power is used by the steering system. In a practical situation, in the high range of engine speeds, such a high level of power consumption does not occur because it is not possible to produce sharp steering movements when travelling quickly. Accordingly, in the high range of speeds of the pump the system maintains a constant condition of power readiness which is not required at that level and which thus results in an unnecessary waste of energy.
In order to overcome that disadvantage, it is already known for the flow control valve to be designed and arranged to provide that the useful flow-pump speed characteristic curve has a falling leg (German published specifications DE-A-No. 22 65 097 and DE-A-No. 26 52 707). The pressure inlet port to the flow control valve is arranged radially in that design, and likewise the relief passage, while the output flow is arranged axially, in the direction of movement of the spool of the flow control valve. The projection portion on the spool is in the form of a valve needle with needle head, the valve needle extending through the axial outlet so as to form a metering orifice whose width depends on the position of the needle head relative to the axial outlet. A disadvantage with that arrangement is that just very small changes in the position of the components result in considerable variations in the cross-sectional area of the axial outlet, through which the flow passes.